A sensitive functional assay has been developed which allows for the quantitation of the three control proteins in serum which regulate the activity of the opsonic C3b site on a cell membrane surface. Using this assay, it has been possible to determine the nature of the biochemical interactions which lead to C3b destruction. The ability of the various C3 fragments to bind to C3b receptors on neutrophils has been studied as has the ability of C5 to bind to the C3d sites. Methods have been developed for the large scale purification and eventual isolation of each of the 18 defined proteins comprising the human complement system. Complement components C3, C5, C7, and C8 have been subsequently purified in high yield to functional and biochemical purity. Complement components Clq, Cl esterase inhibitor, C2, C31NA, beta1H, C4, C6 and C9 as well as the C5b, 6 complex are not yet homogenous and protocols are in progress for their final isolation.